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An Evolutionary Examination of the Floral Display of Catalpa speciosa (Bignoniaceae)

Andrew G. Stephenson
Evolution
Vol. 33, No. 4 (Dec., 1979), pp. 1200-1209
DOI: 10.2307/2407478
Stable URL: http://www.jstor.org/stable/2407478
Page Count: 10
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An Evolutionary Examination of the Floral Display of Catalpa speciosa (Bignoniaceae)
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Abstract

Catalpa speciosa produces inflorescences consisting of many flowers (usually 27), initiates only about three fruits per inflorescence, and matures even fewer fruits. The data from a series of field experiments show that each flower in an inflorescence is capable of developing a fruit when it is the only flower pollinated. However, the probability that a pollinated flower will develop a fruit decreases as other flowers in the inflorescence are pollinated. After approximately four flowers are pollinated per inflorescence, further pollination is inhibited and flowers that open subsequently are functionally male. The remaining flower buds are abscised from the inflorescence. Even though some flowers are functionally male and some buds abscise, the large inflorescence appears to be an adaptation for increasing the fruit set. Data from experiments in which inflorescences were made smaller by the removal of flowers and buds show that the ratio of fruits to flowers decreases with smaller inflorescences. Flowers that are arranged into large inflorescences have a synergistic effect on the number of fruits that are set both per inflorescence and per flower The surplus of flowers in the inflorescence may also serve as a buffer or reserve during years in which pollinators are not available, due to either fluctuations in their populations or poor weather conditions. Because C speciosa blooms for only 8-10 days, it is especially vulnerable to decreases in pollinator activity. The high ratio of flowers to fruits appears to be a compromise between two antagonistic adaptations, one that increases fruit set, and one that limits fruit set. This floral display tends to produce a rather even distribution of small infructescences over the entire tree.

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